1. Field of the Invention
The present invention relates to an apparatus which detects position of a movable body using a magnetic sensor. The apparatus expands a range in which good linearity is obtained. Also, the present invention relates to a vehicle mirror angle detection apparatus using the position detection apparatus.
2. Description of the Related Art
A vehicle mirror angle detection apparatus is incorporated in a so-called remote-control door mirror which allow up-and-down and left-to-right tilt angles of its mirror surface to be adjusted using a motor. The apparatus detects absolute angular position and angular displacement of the mirror surface in up-and-down and left-to-right directions with respect to a predetermined reference angular position. The mirror angle detection apparatus detects a mirror angle adjusted, for example, by the driver and stores it in memory. Subsequently, after the mirror angle is changed, the apparatus is used to automatically return the mirror to its original angular position by reading a setting value out of the memory at the touch of a button. The vehicle mirror angle detection apparatus is also used to detect displacement of a mirror angle in a so-called reverse-shift-activated mirror angle control which allows a driver to visually check areas around rear wheels during backing-up (e.g., when backing into a parking space) by turning a mirror surface of a vehicle outer mirror downward by a predetermined amount simultaneously as gear-shifting means of a vehicle is set to a reverse position and restores the mirror face to the original mirror angular position by turning the mirror surface upward by a predetermined amount simultaneously as the gear-shifting means is subsequently switched to another operating position from the reverse position.
Vehicle mirror angle detection apparatus which use a magnetic sensor such as a Hall-effect sensor have been proposed previously. For example, Patent Document 1 discloses a structure shown in FIG. 2. In FIG. 2, a mirror body 10 consists of a mirror plate 14 fitted in the front face of a mirror holder 12. A pivot 16 protrudes from the rear center of the mirror body 10. A mirror drive unit 17 (mirror angle adjustment actuator unit) is housed and stationarily placed in a front opening of a mirror housing 15 behind the mirror body 10. A universal joint 19 is formed in the front center of the mirror drive unit 17 and the pivot 16 of the mirror body 10 is pivotally coupled to the universal joint 19. The mirror body 10 tilts (swings) around the pivot 16 to adjust the mirror angle.
On the rear face of the mirror body 10, a universal joint 18 is installed away from the pivot 16. A spherical part 20c at an upper end of a screw rod 20 of the mirror drive unit 17 is pivotally coupled with the universal joint 18. The screw rod 20 penetrates a center hole 22a of a gear plate 22. A lower end of the screw rod 20 is slidably led into a guide groove 24 along its axis. The screw rod 20 is restrained from rotating around its axis. With its axial movement restrained, the gear plate 22 is rotatably driven around its axis by a mirror angle adjustment motor (not shown). A spring 22b is installed in the center hole 22a of the gear plate 22. The spring 22b is engaged with a thread groove 20a in the outer surface of the screw rod 20. With this configuration, when the gear plate 22 is rotatably driven by a mirror angle adjustment motor, the mesh between the thread groove 20a and spring 22b causes the screw rod 20 to move linearly in the axial direction with its rotation around its axis restrained. The linear movement of the screw rod 20 causes the mirror body 10 to tilt around the pivot 16, thereby adjusting the mirror angle.
A rod-shaped permanent magnet 26 is embedded in lower part of the screw rod 20 along the central axis of the screw rod 20. The magnetization direction of the permanent magnet 26 corresponds to the axial direction (longitudinal direction). One end in the axial direction constitutes the North (or South) magnetic pole and the other end constitutes the South (or North) magnetic pole. A Hall-effect sensor 28 serving as a magnetic sensor is stationarily placed on the mirror drive unit 17, facing a lower outer surface of the screw rod 20. The axial movement of the screw rod 20 causes changes in that component of a magnetic flux linking the Hall-effect sensor 28 which is oriented in a magnetic sensing direction of the Hall-effect sensor 28, thereby causing the Hall-effect sensor 28 to output a Hall voltage corresponding to axial position of the screw rod 20, i.e., the mirror angle (where the magnetic sensing direction is a direction in which a given element responds to magnetism—i.e., the direction orthogonal to supplied bias current in the case of a Hall-effect sensor and direction orthogonal to the axis of the permanent magnet 26 in the arrangement in FIG. 2). Thus, the mirror angle is detected from the Hall voltage.
Also, Patent Document 2 discloses a configuration of a vehicle mirror angle detection apparatus which uses two permanent magnets 32 and 34 and one Hall-effect sensor 36 as shown in FIG. 3. The permanent magnets 32 and 34 are arranged away from each other and coupled with each other by a coupler 38. The permanent magnets 32 and 34 move linearly in a direction (direction indicated by arrow A) parallel to an arranging direction of the permanent magnets 32 and 34 as the mirror angle is adjusted. Pole faces of the permanent magnets 32 and 34 consist of end faces 32a and 34a fastened by the coupler 38 and end faces (free ends) 32b and 34b on the opposite side. Magnetization directions of the permanent magnets 32 and 34 are opposite to each other. The Hall-effect sensor 36 is located across a gap y of a predetermined length from the end faces 32b and 34b of the permanent magnets 32 and 34. As the mirror angle is adjusted, movement of the permanent magnets 32 and 34 in the direction of arrow A causes changes in that component of a magnetic flux linking the Hall-effect sensor 36 which is oriented in a magnetic sensing direction (direction indicated by arrow B) of the Hall-effect sensor 36, thereby causing the Hall-effect sensor 36 to output a Hall voltage corresponding to the mirror angle. Thus, the mirror angle is detected from the Hall voltage.
[Patent Document 1] Japanese Utility Model Laid-Open No. 64-28343 (FIG. 1)
[Patent Document 2] U.S. Pat. No. 6,382,806 (FIG. 3D)
The mirror angle detection apparatus described in Patent Document 1 provides good linearity in characteristics of the Hall voltage of the Hall-effect sensor 28 with respect to the position of the permanent magnet 26 only in a narrow range. Similarly, the mirror angle detection apparatus described in Patent Document 2 also provides good linearity in characteristics of the Hall voltage of the Hall-effect sensor 36 with respect to the positions of the permanent magnets 32 and 34 only in a narrow range.
Thus, the mirror angle detection apparatus described in Patent Documents 1 and 2 provide good linearity in characteristics of magnetic sensor output (Hall voltage) with respect to the position of the permanent magnet(s) (i.e., mirror angular position) only in a narrow range, leaving no choice but to use the range in which poor linearity is obtained. This causes errors, for example, in detection of displacement in the mirror angle during reverse-shift-activated operation. Specifically, if ΔM denotes a target angular displacement by which the mirror surface is turned downward (e.g., ΔM=6°) and ΔV denotes the amount of change in the Hall voltage per one-degree mirror angular displacement, the reverse-shift-activated operation involves turning the mirror surface downward by the target angular displacement ΔM by turning the mirror surface downward to an angular position where the Hall voltage changes by a set value of ΔM×ΔV from the value of the Hall voltage before the start of the reverse-shift-activated operation. FIG. 4 shows relationship between the mirror angle and Hall voltage at this time. Heavy line C indicates a case where the change characteristic of the Hall voltage with respect to the mirror angular position is an ideal straight line. In the case of the ideal straight line, if the Hall voltage at a mirror angular position M1 before the start of the reverse-shift-activated operation is V1, when gear-shifting means of the vehicle is set to a reverse position and the mirror surface is turned downward until the Hall voltage reaches V2 (V2=V1−ΔM×ΔV) by changing (decreasing)ΔM×ΔV, the mirror surface turns exactly by the target angular displacement ΔM and stops at a mirror angular position M2. This allows the driver to back up the vehicle by visually checking areas around rear wheels properly.
On the other hand, light line D indicates a narrow range of good linearity in the change characteristic of the Hall voltage with respect to the mirror angular position (i.e., poor linearity in the characteristic as a whole). In this case, if the Hall voltage at a mirror angular position M1′ before the start of the reverse-shift-activated operation is V1, when the gear-shifting means of the vehicle is set to a reverse position and the mirror surface is turned downward until the Hall voltage reaches V2 (V2=V1−ΔM×ΔV) by changing (decreasing) a set value of ΔM×ΔV, the mirror surface turns by an angular displacement ΔM′ (e.g., ΔM′=8° if ΔM=6°) larger than the target angular displacement ΔM and stops at a mirror angular position M2′. Thus, the mirror turns excessively, making it difficult for the driver to visually check areas around rear wheels and thereby obstructing the driver in backing up the vehicle.